I wish my mail would go into both programs at once, though. I guess maybe that's not possible. I can't understand why an Apple Mail tech expert would tell me that it would.
Thunderbird — once you've instructed it to do so — only 'taps into' your other email clients and displays your messages, all of which remain on the original servers. The option does exist to delete the messages from the original servers, but you have to select that option (ie, it's not a default). [FYI, just to be on the safe side (as 'backup'), I leave all messages on the original servers and then every so often I log into the them to delete messages I have no use for.]
